Authorities including bomb experts searched an apartment in Revere, Massachusetts, and removed items, after two deadly bombs struck the Boston Marathon. But investigators remained mum about just how the search may be linked to the bombing investigation.

The search late Monday involved a "person of interest," the Revere Fire Department said on its Facebook page. The FBI; the Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives; immigration officials, state and local police, detectives and bomb techs all took part, the fire department said.

The search, just north of Boston, took place with consent, so no search warrant was needed, a federal law enforcement official told CNN.

Tiny clues may help lead to who was behind the terrorist attack that killed three people and wounded 144. Investigators are beginning the painstaking process of piecing through fragments for anything that could indicate the "signature," said a federal law enforcement official who works in the intelligence community.

Unexploded devices that were recovered could also provide a treasure trove of information such as fingerprints and indications of how the exploded bombs were designed, the official said.

There were two such devices that did not detonate, said U.S. Rep. Bill Keating of Massachusetts. One was found near the bomb site at a hotel on Boylston Street; the other was found at an undisclosed location, said Keating, a member of the House Homeland Security committee.

Keating called the bombings a "sophisticated, coordinated, planned attack."

A federal law enforcement official told CNN that both bombs that did explode were small, and initial tests showed no C-4 or other high-grade explosive material, suggesting the packages used in the attack were crude devices.

The FBI is taking the lead in investigating the attack near the marathon's finish line.

"This will be a combined federal, state and local effort," Richard DesLauriers, the special agent in charge of the FBI's Boston Division, said at a news briefing by law enforcement officials on Monday evening.

Describing it a "criminal investigation" that is also "a potential terrorist investigation," DesLauriers said the FBI was declaring federal jurisdiction over the matter through the Boston Joint Terrorism Task Force.

It wasn't clear Monday whether the origin of the bombings was domestic or foreign, according to a federal law enforcement official with knowledge of the investigation. And a state government official said there had been no credible threats ahead of the race.

"People shouldn't jump to conclusions before we have all the facts," President Barack Obama said. "But make no mistake, we will get to the bottom of this, and we will find out who did this."

Obama said that "any responsible individuals, any responsible groups, will feel the full weight of justice."

'A number of active leads'

A law enforcement official in Boston said investigators "have a number of active leads and some good early progress in the forensics analysis" but are yet to identify any suspects.

"The situation remains fluid, and it remains too early to establish the cause and motivation," the FBI's Boston Division said in a statement asking people to call in with any information, images or details related to the explosions.

"No piece of information or detail is too small," it said.

Boston Police Commissioner Ed Davis said many people were being questioned.

In addition to scrutinizing images of surveillance cameras in the area, the FBI was most likely issuing subpoenas for records from cell towers in the area to isolate and trace calls from around Copley Square at the time of the blasts, according to a federal law enforcement official.

As authorities searched the scene, numerous suspicious packages were found, possibly because people fled the area, leaving items behind. Investigators were checking those objects.

Bomb-sniffing dogs were working the area of the bombings and nearby streets, checking every item on curbs or near the street -- including "every construction cone, every Port-a-Jon" -- to make sure there were no explosive devices left, CNN affiliate WHDH in Boston reported.

Law enforcement officials were also chasing possible leads.

Investigators warned police officers to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the Marathon bombs, according to a law enforcement advisory obtained by CNN. The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.

A Saudi citizen with a leg wound was under guard at a Boston hospital in connection with the bombings,

but investigators cannot say whether he is involved at this time, and he is not in custody, a law enforcement official said Monday evening.

After initial suggestions that a third blast Monday, which took place at the John F. Kennedy Presidential Library, could be related to the marathon bombings, police said that that incident was connected to a mechanical problem.